What are some functional differences between orchid mixes?

[Daenerys]

Something I was wondering about. What's important to having a good orchid mix, beyond the size of the bark?

I ask because I bought some 'paph and phrag' mix from repotme.com, which I used on my paphs a few months ago. But then I also used it when repotting my oncidiums and den victoria-reginae (It was the only fine-grained mix I had).

Now, all these orchids seem to have thin roots... but is there any reason why a mix formulated for oncidium would be better than what I used? Or are the differences between such mixes mostly cosmetic?

[Bud]

always go back to the culture of the plant...find out their original environment and what are the situations found on their habitat; some orchids are found thriving on rocks, some are on trees, some are on the second layer of the forest floor with twigs, leaves, rocks and soil ...
also take into consideration if they have tiny roots or big roots...do they need to dry out quick or do they need even moisture?
repotme.com have their mix designed according to how they grow their orchids in their area...make sure your zone can handle that kind of media too

[Tsuchibuta]

Take into consideration your growing conditions and your watering habits as well. For instance if you are heavy handed with the watering can and live somewhere that is fairly humid, you wouldn't want to grow in something that retains a ton of moisture. However if you live somewhere that is dry and you often forget to water then you are going to probably do best with something that retains a lot of moisture.

[tucker85]

The 'Paph and Phrag' mix that you bought will have something in it that will retain moisture. Often sphagnum or peat. It won't hurt you oncidiums as long as you let them dry out completely before rewatering. Oncidiums have fine roots and prefer a little moisture retention. Mixes for cattleyas, laelias or encyclias will be chunky and formulated to dry very quickly. Those orchids like to dry out between waterings.